1. Field of the Invention
This invention relates to a push-button switch for use with a television set, a video tape recorder, a cassette deck or a like apparatus, and more particularly to a push-button switch having a water-proof and dust-proof structure of the face mounted type wherein the inside of an insulating case is enclosed.
2. Description of the Prior Art
In recent years, as miniaturization of electronic appliances progresses, the density of parts mounted on a printed circuit board is progressively increased. This leads to use of a progressively increasing amount of push-button switches particularly of the face mounted type. A push-button switch of the face mounted type is mounted in such a manner that it is first placed on a printed circuit board having cream solder applied to predetermined locations thereof in advance and is then heated in a reflow furnace or the like to solder connecting terminals of the push-button switch to a wiring pattern on the printed circuit board.
An exemplary one of such push-button switches of the face mounted type is shown in FIGS. 7 and 8. Referring to FIGS. 7 and 8, the push-button switch shown includes a heat resisting insulating case 1 having an opening at the top thereof, and a central fixed contact 2 and a pair of opposite side fixed contacts 3 mounted on an inner face of a bottom wall of the insulating case 1. The fixed contacts 2 and 3 extend horizontally outwardly through a side wall of the insulating case 1 to terminate in a pair of connecting terminals 4 and 5, respectively. A dummy terminal 6 is connected to the opposite side fixed contacts 3 and extends horizontally outwardly through the side wall of the insulating case 1 between the connecting terminals 4 and 5. A dome-shaped spring plate 7 serving as a movable contact is placed on and extends between the opposite side fixed contacts 3 such that it is spaced by a little distance from the central fixed contact 2 when it is not operated.
A stem 8 formed from a metal material is mounted for up and down movement in the top opening of the insulating case 1. The stem 8 has a projection 9 provided on a lower face thereof and held in normal contact with the spring plate 7. The stem 8 is covered at an upper face thereof with a water-proof and dust-proof heat resisting film 10 adhered thereto using a bonding agent. A holding plate 12 is mounted on an upper face of the heat resisting film 10 and has an opening 11 formed at a central portion thereof. The holding plate 12 is formed, for example, from a stainless steel plate to which solder plating is applied. The holding plate 12 has a pair of retaining pawls 14 which are engaged with a pair of projections 13 in the form of ribs projected horizontally sidewardly from another pair of opposing side walls of the insulating case 1 to secure the holding plate 12 to the insulating case 1. The holding plate 12 thus defines an upper limit position of the stem 8. An operating portion 15 of the stem 8 is fitted in the opening 11 of the holding plate 12 and is covered with the heat resisting film 10. The holding plate 12 partially extends downwardly to terminate in a ground terminal 18.
In mounting the push-button switch having such a construction as described above, first, the connecting terminals 4 and 5 and the ground terminal 18 of the push-button switch are placed on a wiring pattern 17 of a printed circuit board 16 to which cream solder 19 is applied in advance. Then, the push-button switch and the printed circuit board 16 are heated to a temperature higher than 200.degree. C. in a reflow furnace to melt the cream solder. Consequently, the connecting terminals 4 and 5 and the ground terminal 18 of the push-button switch are connected to the wiring pattern 17 of the printed circuit board 16 by the cream solder 19 as seen in FIG. 8.
If the operating portion 15 of the stem 8 of the push-button is depressed manually, the spring plate 7 is deformed downwardly by the projection 9 of the stem 8 until it is brought into contact with the central fixed contact 2 whereupon the fixed contacts 2 and 3 are electrically connected to each other by the spring plate 7 to turn the switch on. Then, if the depressing force to the operating portion 15 of the stem 8 is removed, the spring plate 7 is returned to its initial upward position by the resilient force of itself thereby to return the switch to its off condition.
In such a conventional push-button switch as described above, the connecting terminals 4 and 5 are soldered by heating at a temperature higher than 200.degree. C. for about 2 to 4 minutes within a reflow furnace as described above. Upon such soldering, air within the insulating case 1 is expanded due to such a high temperature so that the air pressure in the insulating case 1 is raised.
The air of such a high pressure will flow out through a gap between the upper face of the insulating case 1 and the bonding agent on the lower face of the heat resisting film 10 which is reduced in adhesive strength due to the high temperature and also through fine gaps between the insulating case 1 and the connecting terminals 4 and 5. Consequently, a large amount of flux contained in the cream solder 19 will be introduced into the fine gaps around the connecting terminals after the pressure is equalized.
After the push-button switch is taken out of the reflow furnace, the air within the insulating case 1 is contracted and the air pressure is decreased as the temperature lowers. However, because the bonding agent on the lower face of the heat resisting film 10 restores its initial high adhesive strength as the temperature lowers, external air will not be admitted into the insulating case 1 through the bonding agent. At the same time, since flux is filled in the fine gaps 20 around the connecting terminals 4 and 5, external air will not be admitted in through the fine gaps 20 either. Consequently, the air pressure within the insulating case 1 thereafter remains at a very low level. When the air pressure is decreased, the heat resisting film 10 is deformed downwardly to depress the stem 8 until the spring plate 7 is contacted with and stopped by the fixed contact 2. Thus, in the assembled condition of the push-button switch to the printed circuit board 16, the fixed contacts 2 and 3 are held in an electrically connected condition by the spring plate 7. Accordingly, the push-button switch will not operate properly.
In order to prevent this, the dummy terminal 6 is provided to permit external air to flow into the insulating case 1 through a gap 21 between the insulating case 1 and the dummy terminal 6 connected to the opposite side fixed contacts 3 for a long period of time of about 30 minutes to one hour. Even so, since the flux for facilitating application of the cream solder 19 is about 15 percent or so of the cream solder, when melted, the flux may even reach the bottom wall and the side walls of the insulating case 1 and may sometimes stick to portions around the dummy terminal 6. Consequently, air is not communicated into the inside of the insulating case 1 after all, which also gives rise to a problem that the push-button switch will not operate properly.